Acquiring the ability to communicate using natural language and symbolic gestures is a uniquely human capacity that underlies the exchange of information among people. There is as yet no consensus concerning how susceptible this process is to environmental and biological variation. The proposed Program Project focuses on this issue, exploring the extent and the limits of the language-learning process. To examine language growth in the face of environmental variation (Project I), a group of children selected to reflect the demographic distribution within the Chicago area will be observed longitudinally, both at home and at daycare, with an eye toward determining the relation between variations in the speech of caregivers and variations in children's language skills. Assessments will be made of child production and comprehension, and adult input, at 4-month intervals from 14 to 58 mos. Using these data, growth curves will be constructed for each child to track language development across time, and to examine the child's linguistic progress in relation to changes in input. To explore language growth in the face of biological variation (Project III), a group of children with unilateral brain injury will be observed from 14 to 58 mos. with an eye toward describing their language growth, and determining whether environmental variation plays the same role in predicting their growth as it does in children who have not suffered brain injury. Along with traditional measures, two additional probes will be used. (1) The child's communicative competence, and the child's communicative input, will be assessed using gesture as well as speech (Project II). Gesture will be examined in both the brain injured and intact groups to determine whether children who are delayed in speech relative to their peers use gesture to compensate for those delays, and to determine whether the gestures caregivers produce along with their own speech predict individual differences in child language growth. (2) The brain bases underlying communicative competence will be assessed using fMRI techniques (Project IV). The linguistic and gestural skills of individuals who have suffered brain injury at different points in their development will be assessed; fMRI probes will then be used to determine which cortical areas are involved in linguistic and gestural functioning following brain injury occurring at different ages. Three cores provide broad support to the projects: the Administrative Core A, the Data Collection and Transcription Core B, and the Statistical Core C.